If you look at an early front view of #7, the number plate shows two distinct values of black and white indicating two different colors. I like the combination mentioned by Keith.
When I sent up the #10 plate a few years ago, I wanted to make it two tone, but whoever I corresponded with wanted it all black. If you want to see what a two color plate looks like, paint the perimeter around the Vulcan lettering on #10 with artist's tempera paint (water base). Form an opinion, maybe take a few photos and then wash it off.
Most builders plates which I have finished, I painted International red. Don't know if it was protoypical, but it looks nice to me.
Bernie